This is a story I see a lot in the US. Two customers, even with the same carrier, seem to get very different coverage. I thought circa 2010 that I got the same coverage with a prepaid as a postpaid phone but starting around 2018 I would go to New York City and have trouble w/ my prepaid phone.
There seems to be some phone and plan that gets better coverage than another but it is by no means documented. Maybe it just ‘works better the more money you spend’ but without any explanation of what the rules are I am afraid of spending a lot and still getting bad coverage.
I rarely see speeds that fast too; downloading a game pass game is usually 1MB/s for me